Transmission system for stereophonic signals



Dec. 28, 1954 w. w. BOEM-:Ns TAL 2,698,379

TRANSMISSION SYSTEM FOR STEREOPHONIC SIGNALS Filed April 3, 1952 vllllullllllllllllllllllllc||||||ll|||||||||l|||l||ll|llilllx| w. w. BoELr-:Ns ET AL 2,698,379

TRANSMISSION SYSTEM FOR STEREOPHONIC SIGNALS 2 Sheets-Sheet 2 1ST CHANNEL Dec. 28, 1954 Filed April 3, 1952 INVENTORS Willem Wgger Boelens Johannes Jacobus Zoolberg von Zels'r United States Patent 2,698,379 I. Patented Peo. 2 8, 1954 rRANsMrssroN SYSTEM Fon srEREoPHoNIc @sVIGNALs p Willem Wigger Boelens and Johannes Jacobus vZaalherg van Zeist, Eindhoven, Netherlands, assigner-,s to Hartford National Bank and Trust Company, Hartford, Conn., Vas trustee i Application apra 3, 1,952, serial No. 230,414 Claims priority, application Netherlands April 28, 1951 14 Claims. (Cl. 259-6) differences producing primarily the stereophonic eiect.

At the receiver end these coherentsignals A and B are supplied to sound reproducers spaced apart by a certain distance, so that the-desired spatial sound image is obtained.`

The invention relates to a transmission system for stereophonic signals, in which the coherent signals A and B from the sound recorders are emitted as modulations of the same carrier-wave frequency and, at the receiver end, supplied' to separate reproducing devices; and lto ransmitters and receivers for use in this system. For

broadcast purposes this transmission system must frequently fulfil the requirement that the signals outside the frequency band of about 9 kc./s. allotted to the transmitter concerned should by international agreement be attenuated such that broadcast transmitters adjacent in frequency are not disturbed.

The invention has for its object to provide inter alia a stereophonic transmitter, forexample, for broadcast purposes, which may be obtained in a simple manner by completing an existing amplitude-modulation'broadcast transmitter in a manner such that the stereophonic signals emitted by this transmitter can be reproduced Stereophonically by a stereophgonic receiver and that a conventional amplitude-modulation receiver can also provide a substantially undistorted reproduction. of course, no Vstereophonic reproduction is obtained. The invention furthermore provides two types of receivers for stereophoni'c reception.' One type provides excellent stereophonic reproduction and the second type, giving a slightlylower, but satisfactory reproduction quality, is more simple in construction and hence lower in costprice.

According to the invention the transmitter comprises two channels having amplitude modulators connected to a common carrier-wave oscillator and, on the output side, to an aerial system, the emitted oscillations'from the two channels having a phase-shift of 90 relative to the car- Iier-wave oscillation. .The rst channel comprises a sum producer, which adds up the stereophonic signals A and B and an amplitude modulator without carrier-wave sup- .I

pression controlled by the output signal A+B of 'the sum producer and the second channel comprising a difference producer subtracting the stereophonic signals A and B and an amplitude modulator having carrier-wave suppression, controlled by the output signal A-B of the difference 'producer Via an 'integrating network, theV elements' oi' the second channel Vbeing proportioned for a power which is, at the most, ten percent of the power of the corresponding elements of the tirst channel. ln the high-frequency part of the receiver, which has a bandwidth corresponding to that of a normalamplitude-modulation receive;` (about 9 itc/5.) are, connected two channels having an amplitude detector and' a frequency deteetorrespectively,thesum'signal A+B at'the outputm"V the''amplitude'detectorV and' the difference signal A'-B atthe output ofthe frequencyV detector being Vsupplied to in the latter case, f

a sum producer adding up these signals and a difference producer subtracting these signals, the'signals Aand B obtained by addition and subtraction being supplied to separate reproducing devices. 7

In order substantially to preclude a reaction of the two transmitting channels on eachother,4 more particularly, to prevent the high output power of the rst channel from adversely alecting the second channel, the outputs of the two transmitting channels are connected to the aerial system preferably by way of a diplexer, which provides a relative decoupling of thev said outputs.

The receiver for excellent stereophonic reproduction is characterized in that it comprises a device for lfiltering out a carrier-wave oscillation, for example, a selective circuit, the amplitude detector being formed by a mixing stage andthe frequency detector by a further mixing stage comprising a differentiating network included in the output circuit, the selected carrier-wave oscillations being supplied with a relative phase shift of 90? to these mixing stages.

The simple receiver is an amplitude-modulation receiver comprising an additional channel connected to the high-frequency part, having a bandwidth of about 9 lic/s., of the receiver, which comprises the cascade connection of an amplitude limiter and a'frequency detector, the outputs of the two channels beingconnected through the aforesaid sum producer and `difference producerto the separate reproducing devices. l

Consequently, apart from the elements of an ampli- *rude-modulation receiver, the stereophonic'receiver described requires the use of the elements for the additional channel, a sum producer and a diiference producer and an additional loudspeaker.

in order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to Vthe accompanying drawing.

Fig. l shows in schematic form mitter acording to the invention.Y

Fig. 2 shows schematically a( first Vtype of stereophonic receiver according to the invention and Fig. 3 shows schematically a second, simpler type of a stereophonic receiver according Vto the invention.

Fig. l shows a stereophonic' transmitter according to a stereophonic transthe invention, in which the coherennstereophonic signals A and B are taken from two microphones 1 and 2 which are arranged in an artificial head. The low-frequency signals from the transmitter microphones 1 and 2 occupy, as a rule, a frequency range'of, for example, 20 to 9000 c./s.

The transmitter comprises two channels 4 and 5, controlled by the signals A and B from the transmitter microphones 1 and 2, the emitted oscillations from the two channelshaving a phase shift of 9C, relative to the carrier-wave oscillation. The rst channel 4 comprises a sum producer which adds up the stereophonic signals A; and B and which is connected in cascade to a voltage amplifying stage 6, a pre-amplifying stage 7 and a modu-l lation amplifier 8, which is formed, for example, by a grid-current controlled class B push-pull amplier. The amplified microphone signals appearing across the output circuit of the modulation amplifier 8 are supplied through an output transformer 9 as modulating voltages to an anode modulator operating without carrier-wave sunpres sion. The anode modulator comprises a` class-C-connected triode 10, the anode of which is connected via a blocking capacitor 11 to an output circuit 12, while the control-grid is connected via a high-frequency choke 13 to the negative terminal of a grid-voltage supply 14. The anode of the triode 10 is connected through a high-frequency choke 1S and theV secondary winding of the output transformer 9 to an anode-voltage supplyl and the highfrequency oscillations to be amplified and derived from an oscillator 17 are supplied via a power'amplifier 18 to the control-grid. The coil 19, coupled with the output circuit 12, is connected to an aerial 21 via a network 2Q, which will be described more fully hereinafter. 1

'The second transmitter channel 5 comprises a difference producer subtracting the stereophonic signals A and B, the output signal A-B of which is supplied to`an intecouplingrof the said output.

grating network 22, which is connected in cascade with a voltage-amplifying stage 23, a pre-amplifying stage 24 and a modulation amplifier 2S. The modulation amplifier 25 provides the modulation voltage of an anode modulator operating with carrier-wave suppression through the intermediary of an output transformer 26. This modulator comprises two class C-connected triodes 27 and 27' in push-pull, of which the anodes are connected through blocking capacitors 28 and 28 to different ends of an output circuit` 29, and the control-grids are connected through high-frequency chokes 30 and 30 to the negative terminal of a grid-voltage apparatus 31.Y The anodes of the triodes 27 and 27 are connected through high-frequency chokes 32`and 32 to the ends of the secondary winding of the transformer 26, a central tapping of which is connected to the terminal of an anode-voltage apparatus 33 and the high-frequency oscillation to be-modulated, initiating from the oscillator 17 and ampliied by the energy amplifier 18 are supplied through coupling capacitors 34 and 34 in co-phase to the controlgrids. The amplitude-modulated oscillations with suppressed carrier-wave occurring across the output circuit 29`are supplied through a coupling coil 35 and the coupling network 20 to the aerial 21.

The sum yproducer and the differenceproducer, which form the inputs of the two transmitter channels, comprise in the embodiment shown two transformers 36 and 36', connected on the primary side to the individual microphones 1 and 2, vthe output terminal 37 of the transformer 36 being connected to a central tapping of the transformer 36'. The sum signal occurring between the terminals 38 and 39 is taken from a resistor 40 and the difference signal, occurring between the terminals 38 and 41,*is supplied to the integrating network 22. In order to load equally the sum producer and the difference producerthe integrating network 22 comprises- Van ohmic input impedance of a value approximately equal to that of the resistor 40 throughout the frequency range to be transmitted. In the embodiment describedy the integrating network therefore comprises two parallel-con- ,nected branches, of which one is formed by the seriesconnection of the choke 42 and a resistor 43 and the other Vby the series-connection of aresistor 44 and a capacitor the control-grids of the triodes 27 and 27 comprises a 90 phase-shifting network 46. This is not strictly necessary; as an alternative, for example, one of the output circuits of the amplitude modulators 10, 27 or 27 may be provided with a delay line'102, the electrical length of which is equal to one quarter wavelength of the common carrier-wave oscillation. y

In the transmittmgdevice described the elements of Vthe channel 5 are proportioned-for a power of not more than 10% (in thefembodiment shown, for example, 2%) of the power of the corresponding elements of the channel 4. In order to..preclude substantially a troublesome reaction of the modulator 10fon the modulators 27 and Y27', where the output circuits 12 and 29 are connected tothe aerial 21,-these circuits are connected to the aerial 21 through a diplexer 20,- which provides a relative de- Such diplexers are 4known per se and comprise, in principle, a bridge circuit,chaving Y for example'three'inductive, and a capacitative impedance,

the outputs of theamplitude modulators 10, 27 and 27' being connected to diagonally opposite points of the bridge circuit, the other diagonal points beingconnected tothe aerial 21` and an imitation impedance. The decoupling between the channels 4 and 5 may, as an alternative, be obtained, for example, by connecting the output Vcircuits 12 and 29 to different aerials, the positions and radiationcharacteristics of which are chosen to be efficient for the purpose.V

Y The proportioning of the elements ofV the. channel 5" for a power which is appreciably lower than the power of the corresponding elements of the channel 4 is made possible by the combination of a number of measures which can be taken with Vregard to-the stereophonic transmission,

For Vthe lower signal frequencies to about 250 and 350 c./s. whlch normally occur with comparatively great amplitude in the sound image, an appreciable decrease in the signal-voltage amplitude to be transmitted is obtained in the embodiment described by supplying the coherent signals A and B from the microphones 1 and 2 to'a difference producer. The difference signal A-B produced exhibits a low amplitude value for the said low frequencies with respect to the A-i-B signal, since for these frequencies the signals occurring at the microphones 1 and 2 have onlyV small intensity and phase differences.

For the frequencies over 500 to 700 c./s. no appreciable amplitude reduction Voccurs owing to the production of the A-B signal, so that the amplitude maximum of the amplitude-frequency characteristic of the sound signals for the difference signal A-B does not lie at about 500 c./s., as is the case with the signals A, B or A+B, but shifts to higher frequencies, for example, 700 to 800 c./s. It should be noted that the maximum amplitude values for the frequencies of 700 to 800 c./s. are lower than those for frequencies of about 400 to 500 c./s. Thus the signal amplitudes to be passed through the A-B channel 5 are smaller than those of the A+B channel 4 and the A-B channel may thus be proportioned for a lower power than the A-I-B channel. However, takingV only the difference producer into consideration, the output power required for the A-B channel must be at least 10% of the output power of the A-l-B channellin order to obtain a reasonable transmission in genera Y Y With stereophonic reproduction high signal frequencies of for example 3000 to 4000 c./s. and more play a comparatively unimportant part with respect to the stereophonic effect and they may even give rise to interfering stereophonic effects.

In other words, for sterophonic reproduction particularly signal frequencies from about 200 to '300 c./s.to. about 3500 to 4000 c./s. are important and for higher signal frequencies their importance for the sterophonic-l According to the invention this condition is utilised by supplying the A-B signal through a signal-fre-qweuciesY integrating network, of which the time constant (or the limit frequency) is chosen to be suitable, i. e. such that the aforesaid maximum at about 700 to 800 c./s. in the amplitude frequency characteristic for the A-B signal'is considerably reduced or shifted to a lower frequency range. By using the `integrating network in the A-B channel 5 the signal level to be passed through this channelV is restricted in a manner such thatv for the A-B channel, at least when a modulator operating with carrier-wave suppression, is used, an output power of less than 10% of that of the A+B channel is sufficient. Y l

The high-frequency power required for the A-B channel 5 varies with the limit frequency of the integrating network 22; it decreases with a decrease in limit frequency. Of course, the limit frequency cannot be decreasedA at will. Provision must be made for a reasonable signal-noise ratio for the A-B signal for the frequencies which are important for the stereophonic effect. ln practice a suitable value of the limit frequency of the integrating network is, for' example,'200 to 300 c./s.

The integrated` difference signal modulates a carrier wave in amplitude in the A-B channel with carrier-wave suppression, this carrier-wave having a phase shift of 90 with respect toV the amplitude-modulated carrier-wave in Y the A+B channel without-carrier-wave suppression. As-

suming the construction of the two amplitude modulators to be suitable, no sideband frequencies outside the amplitude-modulation spectrum of about 9 kc./s. are produced in the two channels. This also applies to the modulation Y f spectrum after the output-oscillations of the A+B chanwave of the A+B channel, i. e. a frequency modulation,

since the difference signal A-B is supplied through an integrating network to the modulator in the A-B chan'- Vnel. As will be obvious after the remarks on the modu-V lation spectrum, we are concerned here with afrequency Y modulation `having a very small sweep (narrow b and fre- Vquency modulation), which is otherwise not identical with normal frequency modulation owing to the com- .85 plete absence of Vhigher-order sideband frequencies, which is of importance for the construction of the receiver, as will be explained more fully hereinafter.

At the receiver end the A+B signal can be obtained by means of a conventional amplitude detector. If the A+B channel of the transmitter has not given a carrierwave component, the A+B sideband oscillations will substantially not contribute to the output signal of the amplitude detector, to which all emitted oscillations are supplied. Consequently, the stereophonic transmission may be received Without objection by means of a conventional amplitude-modulation receiver, although, of course, without stereophonic reproduction.

If the transmitter were constructed such that the carrier-wave is suppressed in the A+B channel and that only the carrier-wave of the A+B channel is emitted, an amplitude detector at the receiver end would produce only the A+B signal. Similarly with the construction of the transmitter described above frequency detection of the emitted oscillations produces only the A+B signai and in the case of carrier-Wave suppression in the A+B channel only the A+B signal.

As soon as the two'transmitter channels contribute to the total emitted carrier-wave, both amplitude detection and frequency detection of the emitted oscillations supply' a mixture of the A+B signals and the A+B signals, in other words, cross talk occurs between the A+B channel and the A+B channel in a measure which varies with the phase of the total carrier-wave with respect to, for example, the carrier-wave component of the A+B channel only.

In order to reduce such cross talk, which also interferes with non-stereophonic reception, in the transmitter construction shown, to an interference free level, it is necessary to provide that the carrier-wave component across the output of the A+B channel is substantially negligible with respect to the carrier-wave component supplied by the A+B channel. This is obtained', since the power of the carrier-wave supplied to the A-B channel is only a fraction of that of the carrier-wave of the A+B channel and since, furthermore, carrier-wave suppression is effected in the modulator in the A+B channel.

Another form of cross talk, which is particularly of importance for stereophonic transmission, is constituted by the relatively unequal phase course for the low-frequency signals in the transmission channels, which are understood to comprise the associated transmitter and receiver channels. For a reason to be described more fully with reference to the receivers, the contributions of the transmitter channels to this difference in the course of the phase are preferably a small fraction, for example, ln of the maximum difference permissible in the complete system. It is particularly' for this reason that the loads of the sum producer and the didi rence producer are eoualized throughout the frecuency range to be transmitted by means of the resistor 40 and the network 22.

it should nally be noted that recovery of the carrierwave oscillation at the receiver end to ensure excellent ster-eophonic reproduction is rendered difficult by the frequency modulation of the emitted carrier-.wave osciiiations, owing to the low frequencies of the difference C signals. Also with regard to the fact that these frequencies contribute only little to the stereophonic effect, they may be completely suppressed by means of a high pass iilter iii having a limit frequency oi", for example, 200 to 300 c./s., included in the lou/frequency part of theA-B channel.

'Fig'. 2 shows the type of receiver according to the invention for perfect stereophonic sound reproduction of signals transmitted by a transmitter as shown in Fig. l.

The high-frequency signals received through an aerial 47 are supplied to a mixing stage 49, connected to a loca! oscillator 4S, whose output circuit, having'usual bandwidth of about 9 lic/s. is constituted by a bandpass iilter :30. The latter is connected 'm parallel with the inputs ofA three channels 51, 52 andl 53, which contain inthis order an amplitude detector, a device jorY filtering out the carrier-wave oscillation and a frequency detector.

The channel 52, which serves to iiiter out the carrierwove oscillation, comprises a circuit 54 tuned to the carrier-Wave frequency Vand constituting the output circuit of a pentodc 55, connectedas an intermediate-frequency amplifier to the bandpass filter Si?. The Vcircuit 54 is negativeiy damped, since it is included in the anode circuit of a. pentode 56, the controi-grid of which is regeneratively coupled with the circuit 54 by Way ofa-coi 57; By suitable adjustment of' the degreeof feedh'ackfa very narrow bandwidth may be obtained for the circuit 54.

Since the receiver must be tuned veryaccurately, vthe use of automatic frequency correction (A. F. C.) is highly desirable for practical reasons.

it should be noted that the carrier-Wave without Sideband frequencies may be obtained at the receiverV end, as an alternative, not by means of filtering, but, for example, by means of a local carrier-wave oscillator, of which the frequency and the phase are stabilised by means of an A. F. C.circuit on the incoming carrier-wave oscil lation. T he incoming carrier-Wave isv then supplied, for example, through a circuit tuned to the carrier-waveV to mixer operating as a phase detector, in which-it is compared with an oscillation from the local carrier-'wave oscillator, the output voltage of the mixer being supplied through a lowpass lter asa control-voltage to a fre- Y quency corrector coupled with theY local oscillator.

The channel 51 comprises a mixer formed by a pentode e8, operating as an amplitude detector, the incoming signais being supplied to the control-grid and the filtered carrier-wave from the selective circuit 54 to the suppressor grid. Since the filtered carrier-wave is in co-phase with the incoming carrier-wave for the sum signal A+B, the low frequency sum signal A+B is produced across the output circuit of the mixer 58, constituted by a resistor 59 and then ampliiied in a low-frequency amplifier 60.

The channel 53 comprises a frequency detector having a mixer, for example, a hexode or a Vpentode. In the embodiment shown the mixer is formed by a pentodc 61, the anode circuit of which comprises a diierentiating network constituted by the series combination of a coil 62 and a resistor 63. The incoming signalsqfrom the bandpass filter are supplied to the control-grid of the pentode 6i and the suppressor grid is connected through a 90 phase-shifting network to the resonant circuit 54. in the embodiment described above the phase-shifting network is constituted by a resonant circuit 64, connected inductively to the circuit 54 andtuned to -the carrierwave frequency. Since the mixed voltage supplied tothe suppressor grid of the pentode 61 corresponds in frequency and in phase with the carrier-Wave ofthe in,- tegrated difference signal A+B, emitted in amplitudemodulation with carrier-wave suppression, an anode current is produced in the pentode 61, which varies in the rhythm of the integratedV diiference'signal A+B. Consequently, a low-frequency voltage is produced across the direrentiating network 62, 63, the value of this vol tage corresponding with the initial low-frequency difference signal A+B. This is supplied to a low-frequency amplifier 65 for amplification.

Since the demodulation process in the receiver shown in Fig. 2 is exactly reciprocal with the vmodulation process carried out at the transmitter end, an excellent reception of the A+B signals and of the A+B signals may be obtained. In order to obtain stereophonic sound reproduction, the sum signal A+B and the difference signal A+B occurring across the outputs of the low-frequency ampliers 69 and 65 respectively are supplied in common to a sum producer adding up these signals and a subtracting difference producer, the coherent signals A and B obtained by addition and subtraction being supplied, if desired, through separate volume control circuits, to separate reproducing: devices 66 and 67.

The sum producer and the ditference producer are constituted by transformers 68 and 69 respectively, the primary sides of whichV are connected to the outputs of the low-frequency amplifiers 60 and 65, an output terminal 70 of the transformer 68 being connected vtoV a central tapping of the transformer 69. The two stereophonic signals A and B are taken from the other terminal 11l of the transformer 63 and the outer terminals 72 and 73 of the transformer 69 respectively.

if, in the device decribed above, the amplitude ofthe carrier-wave oscillation supplied to the suppressor grids of the mixers S and 61` is suiiciently greatV withrcspect to the signals supplied to the control-grids andv if it has the correct phase, the output voltages of the mixing stages reproduce accurately the sum signal A B and the difference signal A+B.

Fig. 3 shows the type of a simple stereophonicreceiver according to thel invention.

in this embodiment the signals. received through the aeriai 74 are supplied to a mixing stage 76, connected to a carrier-wave oscillator 7S; the output circuit ofthis stage is designated by 77. "The circuit 77 is coupled coupled Atransformers.

. networks comprising.resistors.v l I Y quency amplifier 82, the output bandpass filter 83 of which is connected to adiode detector 84, having anV output impedance 85.,

The channel 81 comprises the cascade connection of :an amplitude limiter and a frequency detector.v The amplitude limiter comprises a pentode 86, as an intermediate-frequency amplifier, the control-grid being connected through a resistor 88, shunted by a capaci- :tor 87, to the cathode, while the output circuit is con- :stituted by a bandpass filter 89, comprising the circuits 90 ,and 91, which form part of a frequency detector. The vfrequency detector is of a type known per se for detection of normal frequency-modulation transmission and comprises twoA rectiiers 92 and 93,'connected to the ends of the circuit 9.1 and interconnected by an output impedance 94. A central tapping of theoutput impedance 94 is connected through a high-frequency choke 95 to a central tapping of the circuit 91, which is connected through a Y blocking capacitor 96 to an end of the circuit 90.

If the signals modulated in amplitude by the sum signal 4A-lV-'Band the signals modulated in frequency by the difference signal A-B occur across the output circuit 7 7 of the intermediate frequency stage 76, having a normal bandwidth of about 9 ken/s., the low-frequency sum signal ATI-B is producedV across the output impedance 85 of the diode vdetector v84, as will be obvious without further explanation.

e The oscillations from the circuit 77 produce across the network 87, 88 of the amplitude limiter 86, included in lthe control-grid circuit a grid bias voltage varying with the sum signal A+B, supporting a limiting effect of the pentode 86. If the uetworkr87, 88 is eiciently proportioned, ,anV oscillation modulated in frequency by the difference` signal is produced across the output bandpass filter 89'of the amplitude limiter, the amplitude modulation of these oscillations being largely suppressed. The low-frequency difference signal A-B then occurs across the output impedance 94Yof the frequency detector.

It should be noted that an additional amplitude limiter Y may be connected in cascade with Vthe amplitude limiter 86.

As 'stated with reference to the A+B transmitting channel, the A-B sideband Vsignals represent, owing to the absence of higher-order sideband frequencies, no normal angular modulation of the emitted carrier-wave; by using a strong limitation or amplitude modulation feedback in the receiving channel distortions otherwise involved in the use of a conventional frequency-modulation detector are avoided to al great extent.

In order to obtain stereophonic sound reproduction the signals occurring across the output impedances 85 and 94 of the Vamplitude detector and the frequency detector are supplied in a manner similar to that described with reference to the receiver shown in Fig. 2 through a sum producerl and a difference producer to separate sound reproducers 66 and 67. Corresponding elements are designated by the same reference numerals.

In the receivers described above the coherent signals A and B are obtained by addition 'and subtraction of the sum signal A+B Vand the difference signal A-B. If the transmission channelsfor the low-frequency signals are not equal in phase, cross talk occurs owing to the aforesaid, addition and subtraction.' It has beenrfound by lexperiment that with stereophonic transmission an appreciable valueY of cross-talk (for example, a cross talk level of +15 dbs to -20 dbs) is permissible. Since the transmitter channels, as is described above, have a very small relative phase difference, a considerably greater tolerance is permitted at the receiver end. This is an important advantage with the mass production of the receivers.

'In the embodiments described above the sum producer andthe diiference producer at the transmitter end and at the receiver end are constituted byV two mutually These sum producers and differenceproducers may, as an alternative, be constructed in'a different way, for example, by means of two lowfrequency amplifiers, separated at the output, one signal being supplied in Vco-phase vand the other signal in phase opposition to the control-grids of the amplifying tubes. As an alternative, use may be made. for this purpose, of

connected It will be obvious that in the receivers described above either the A-B branch or the A+B branch may be made inoperative, if only reception of amplitude-modulated or of frequency-modulated signals is aimed at.

What we claim is:

l. In a stereophonic system wherein coherent stereophonic signals A and B are sent out as modulations on a common carrier; a transmitter comprising rst and sec-l ond channels, Vthelirst channel including a sum producer for addingV the signals A and B to produce an output signal A+B and a first amplitude modulator coupled to the output of said sum producer, said second channel including a difference producer subtracting signals A and B to produce an output signal A-B, a carrier-wave suppression second amplitude modulatorV and an integrating network coupling the output of said difference producer to said second modulator, a carrier wave oscillator to said first and second modulators, means to impart a 90 phase displacement to the carrier wave conveyed in the iirst channel relative to that in the second channel,.said second channel being proportioned to produce a power output not exceeding 10% of the power output of the rst channel, and an antenna system coupled to the outputs of the first and second channels; and a receiver comprising two channels having an amplitude detector and a frequency detector respectively, the amplitude detector yielding the sum signals A+B and the frequency detector the difference signals A-B, a sum producer and a difference producer coupled in common to said detectors to recover the coherent signals A and B, and means coupled toY said producers to reproduce separately said coherent signals. Y i

2. A system, as set forth in claim Vl, wherein the channels in said transmitter and-the channels in said receiver have corresponding phase relations for low-frequencyV signals.

3. In a stereophonic system wherein coherent stereophonic signals A and B are sent out as modulations on a common carrier; a transmitter comprising first and second channels, the first channel including a sum producer for adding the signals A and B to produce an output signal A-l-Band a rst amplitude modulator coupled to the output of said sum producer, said second channel including a difference producer subtracting signals A and B to produce an output signal A-B, a carrier wavesuppressing second amplitude modulator and an integrating network coupling the output of said difference producer to said second modulator, a carrier wave oscillator coupled to said first and second modulators, means to impart a 90 phase displacement to the carrier wave conveyed in the rst channel relative to that in the second channel, said second channel being proportioned to produce a power outputnot exceeding 10% of the power output of the first channel,V and -an antenna system coupled to the outputs of the rst and second channels.

4. A stereophonic transmitter, as set forth in claim 3, wherein said antenna system coupled to the outputs of the first and second channels is constituted by a common aerial, and further including a dipleXer for coupling said common aerial to both of said channels.

5. A stereophonic transmitter, as set forth'in claim 3, wherein said means to impart a 90 phase displacement comprises a 90 phase shifting network interposed between said oscillator and one of the modulators.

6. A stereophonic transmitter, as set forth in claim 3, wherein said means to impart a 90 phase displacement comprises a delay line in one of the channels coupled to the output of the modulator therein and having an electrical length equal to an odd number of quarter wavelengths vof the carrier wave oscillation.V

7. A stereophonic transmitter, as set forth in claim'3, wherein the integrating network in the second channel possesses a limit frequency of between 200 to 3007cycles substantially. Y

8.'A stereophonic transmitter. as set forth in claim 3, further including a high pass filter connected in series with the integrating network and having a limit frequency of between Y200 to 300 cycles substantially.

9. A stereophonic transmitter, as set forth in claim 3, further includinga resistor for loading said sum producer and wherein ythe integrating network loading the difference producer .has a constant resistiverinput impedance throughout the transmitted range, the value ofthe impedal-ice corresponding with the value of said load resistor. 105A stereophonic transmitter, as set forth in claim 3, Ywherein said sum'producer and 'saidldifterence .producer` are constituted by two transformers each having a primary and a secondary, means connecting one terminal of the secondary of one transformer to the center point of the secondary of the other transformer, means to apply the signals A and B to the respective primaries ofthe two transformers, means to derive the sum input Afl-B from the other terminal of the rst transformer and one of the 1output terminals of the second transformer respective y.

1l. In a stereophonic system, as set forth in claim 3, further including a receiver provided with a highfre quency stage having a bandwidth corresponding to that of a normal amplitude-modulation receiver not exceeding 10 kilocycles, comprising two channels having an amplitude detector and a frequency detector respectively connected thereto, the amplitude detector yielding the sum signals A+B and the frequency detector the difference signals A-B, a sum producer and a diterence producer coupled in common to said detectors to recover the coherent signals A and B, and means coupled to said producers to reproduce separately said coherent signals.

12. A stereophonic receiver, as set forth in claim l1, wherein said amplitude detector is constituted by a first mixing stage and said frequency detector by a second mixing stage whose output circuit is provided with differentiating network, means to filter out the carrier wave oscillations from the incoming signal, and means to apply the ltered signal to the rst and second mixing stages with a relative phase displacement of 90 whereby the signal applied to the first mixing stage is in phase quadrature with respect to the signal applied to the second mixing stage.

13. A stereophonic receiver, as set forth in claim 11, wherein said two channels are constituted by an amplitude modulation receiving channel provided with a high-frequency stage having a bandwidth of approximate- 1y 9 kc./s., the amplitude modulation detector being coupled thereto, and an additional channel coupled to said high-frequency stage and including an amplitude limiter, the frequency detector being coupled to the output of said limiter.

14. A stereophonic recevier, as set forth in claim 11, wherein the sum producer and difference producer are constituted by tirst and second transformers each having a primary and a secondary, the primaries of the transformers being connected respectively to the outputs of the amplitude and frequency detectors, one terminal of the secondary of the first transformer being connected to the center point on the secondary of the second transformer, the signals A andB being taken from the other output terminal of the rst transformer and the output terminals of the second transformer respectively.

References Cited in the tile of this patent UNITED STATES PATENTS Number Name Date 1,666,158 Affel Apr. 17, 1928 FOREIGN PATENTS Number Country Date 540,185 Great Britain Oct. 8, 1941 OTHER REFERENCES Binaurel Transmission on a Single Channel, by Eastman and Woodward, Electronics, February 1941, pages 34 to 36. 

